In vivo chronic nicotine exposure differentially and reversibly affects upregulation and stoichiometry of ?4?2 nicotinic receptors in cortex and thalamus

Studies with heterologous expression systems have shown that the a4b2 nicotinic acetylcholine receptor (nAChR) subtype can exist in two stoichiometries (with two [(a4)2(b2)3] or three [(a4)3(b2)2] copies of the a subunit in the receptor pentamer) which have different pharmacological and functional properties and are differently regulated by chronic nicotine treatment. However, the effects of nicotine treatment in vivo on native a4b2 nAChR stoichiometry are not well known. We investigated in C57BL/6 mice the in vivo effect of 14-day chronic nicotine treatment and subsequent withdrawal, on the subunit expression and b2/a4 subunit ratio of 3 Heepibatidine labeled a4b2*- nAChR in total homogenates of cortex and thalamus. We found that in basal conditions the ratio of the b2/a4 subunit in the cortex and thalamus is different indicating a higher proportion in receptors with (a4)2(b2)3 subunit stoichiometry in the thalamus. For cortex exposure to chronic nicotine elicited an increase in receptor density measured by 3 Heepibatidine binding, an increase in the a4 and b2 protein levels, and an increase in b2/a4 subunit ratio, that indicates an increased proportion of receptors with the (a4)2(b2)3 stoichiometry. For thalamus we did not find a significant increase in receptor density, a4 and b2 protein levels, or changes in b2/a4 subunit ratio. All the changes elicited by chronic nicotine in cortex were transient and returned to basal levels with an average half-life of 2.8 days following nicotine withdrawal. These data suggest that chronic nicotine exposure in vivo favors increased assembly of a4b2 nAChR containing three b2 subunits. A greater change in stoichiometry was observed for cortex (which has relatively low basal expression of (a4)2(b2)3 nAChR) than in thalamus (which has a relatively high basal expression of (a4)2(b2)3 nAChR).